1. Field of the Invention
This invention relates to an image processing apparatus and method, and more particularly, to an image processing apparatus and method for performing spatial frequency characteristic conversion, such as smoothing or sharpening processing, on an input image signal.
2. Description of the Related Art
In order to digitally read a color image by decomposing it into three color components, a flat-bed-type original reading apparatus using a one-dimensional line sensor, or a camera-type image pickup apparatus using a two-dimensional area sensor, is generally utilized.
However, an image signal output from such an apparatus includes an analog noise component from the sensor, and random noise generated due to the characteristics of an A/D (analog-to-digital) conversion circuit for converting a sensor output into a digital signal. Such noise degrades the quality of the obtained image. For removing the noise, smoothing processing in which the value of a target pixel in the image signal is replaced with the mean of the value of the target pixel and the values of pixels in the vicinity of the target pixel, is effective. However, such smoothing processing degrades sharpness of edge portions where gradation of the image abruptly changes, thereby degrading image sharpness.
An attempt to correct degradation of an image by removing noise by means of smoothing processing utilizing the spatial distribution or statistical characteristics of image data is now under way. For example, a method for removing only noise without degrading edges, by means of a smoothing filter called a median filter applied to the entire image, and a method for removing noise by performing smoothing processing for other regions than edge portions (regions where gradation gradually changes) by determining attributes of the image, have been proposed.
However, the above-described techniques have the following problems.
In the conventional methods, an edge portion is discriminated from a flat portion utilizing the signal distribution characteristics of an image, and filtering processing is (selected) based on the result of the determination. However, it is impossible to completely prevent occurrence of erroneous determination for an edge portion and a flat portion, and therefore it is inevitable that some noise remains, or degradation of edge portions occurs, to some extent.
The above-described problems also arise when performing sharpening processing of an image, such as edge emphasis or the like.